When positioning a land-based radar which purpose is to scan the horizon around a geographical site, it is often practical to know at which elevation to start scanning. This way one can avoid spending time scanning directly into e.g. mountains.
One possible solution is for the radar to initiate a search in elevation (e.g. from the highest allowed elevation and down) for the horizon, typically on a sector-by-sector basis, and detect where the ground clutter level starts to be significant.
A radar can get a certain amount of false echo above the horizon (e.g. birds, second-time-around echo from distant planes, etc) which may set the horizon higher than necessary. Sometimes radar echo are also sufficiently absorbed by surrounding terrain so that the horizon may be set lower than required.
Rain, snow and fog can also affect such a measurement.
In addition a radar often has a minimum range of detection. This result in objects which constitute part of the horizon and being “too close” blocks the view without the radar detecting this.
Another important issue in military applications is Electronic Warfare where radar silence is an important counter measure. This means that search elevation have to be found without using the radar actively.